In both copier/duplicator systems and duplicator/printer systems there is need for collecting the copy sheets in organized stacks at the output of the equipment. For this purpose, various types of sheet collecting and stacking structures are available for attachment at the output of the duplicator equipment. Typically, such structure can function to either collect copy sheets in a single uncollated stack or to collect the sheets in separate bins with a single collated set in each bin. In this latter mode of operation, the copy sheets are stacked in their proper original/copy sequence. This is accomplished by stepping the sheet feeding means into alignment with the entrance to each bin to serially feed the sheets into the bins, the sheet feeding means moving from bin to bin as each single sheet is fed to the bin.
As sheets are stacked in the receiving bin or bins of the collecting and stacking apparatus, their trailing edges adjacent the entrance to the bin frequently tend to curl up. This quickly raises the height of the stack within the bin. With a limited access entrance to the bin, this curling of the trailing edges forms an obstruction at the entrance so that the next sheet fed to the bin will be blocked from entering. This, in turn, can cause damage to the sheets or even jamming of the apparatus.
Different devices have been developed to alleviate this condition and to provide a clear, unobstructed entrance to each bin as each sheet is fed therethrough. Mechanical devices of this type have included mechanisms for continuously engaging the trailing edge of the accumulated stack of sheets at the entrance to each bin to keep the entrance clear, multiple hold-down bars operating sequentially as each sheet is fed to the stack, and camming mechanisms for opening or widening the entrance to the bin as each sheet is fed therethrough. Pneumatic devices are also available for holding down the trailing edge of the stack. These devices employ means for directing an air blast at the trailing edge of the stack to keep all sheets lying flat.
Devices of the types described above possess certain disadvantages. Some are not positive acting in that they do not provide a positive clearance of the entrance. Devices which are based on blasting air or widening the entrance are examples. With pneumatic devices, the air blast also has the disadvantage of obstructing the free feeding of the sheet which, to some degree, must run counter to the air blast. Other devices actually provide a mechanical obstruction to the feeding of incoming sheets and the sheets are relied upon to open the obstruction.